Metallic Crystals - The Structures of Pure Metals

Structures of Pure Metals

Metallic crystals are made of metal cations in fixed lattice positions
and delocalized electrons that form the metallic bonds, holding the lattice together (metallic bond). Because pure metals only contain one type of lattice component, high coordination numbers
are possible.

Most metals crystallize in one of the two closest-packed
arrangements or in a body-centered cubic lattice.

The closest-packed arrangements of spheres are formed from stacks of planes that are in turn
filled with spheres packed as densely as possible.

Fig.1

Closest-packed layer

In a closest-packed layer, every sphere is surrounded by six neighbors. Between every three
touching spheres is a hole. The number of holes is twice the number of spheres: every sphere
is surrounded by six holes that each belong to three spheres; each sphere is thus allotted
6÷3=2 holes. These holes can be filled with spheres from the next layer. However,
because the second layer cannot contain more spheres than the first, only half
of the holes of the first layer are filled with spheres from the second. The other
half of the holes remain unoccupied. In turn, the second layer, which has an identical structure to the first, has twice as many
holes as spheres for the spheres from the third layer to occupy.

The third layer may:

be located directly over the first

or be shifted relative to both the first and second layers.

These possible arrangements result in the two primary types of closest-packed structures of spheres: hexagonal closest-packed structure and cubic closest-packed structure.